Coronary ischemia is a disease caused by the reduction in blood flow in the coronary arteries. Some forms of coronary ischemia are particularly severe, leading to acute coronary syndrome (ACS), which is a dangerous condition often caused by the rupture of plaque in the arteries which leads to an obstruction to the blood flow down the artery. Other forms of coronary ischemia are more chronic, in which the coronary artery gradually narrows over time from atherosclerosis, causing the blood flow down the coronary artery to slowly decrease. This type of ischemia is often less dangerous to the patient, as the heart is able to compensate by creating collateral arteries.
Nationwide, more than 6 million emergency department (ED) visits a year involve patients presenting with chest pain, accounting for more than 5% of all ED visits. Many of these patients are admitted to the hospital for signs of ACS. However, more than 60% of the patients admitted to the hospital with chest pain do not have acute coronary syndrome ACS.
Despite the over-triage of patients (with economic implications of over $8 billion in annual costs), the rate of missed diagnoses of hospitalized patients with ACS remains unacceptably high (2% to 8%) with concordant serious public health consequences. Indeed, the significant morbidity and mortality associated with missing a myocardial infarction is the highest overall cost to insurers of any missed diagnosis in emergency medicine.
Furthermore, there is no simple established test for validating whether a person with chest pain has a low enough risk of ACS to be discharged from the hospital. Rather, patients deemed to be at increased risk are subjected to a variety of non-invasive tests including exercise treadmill testing, nuclear scintigraphy, coronary computed tomography angiography (CTA), or a myocardial perfusion single photo emission computed tomography (SPECT) test.
Virtually all of these non-invasive tests involve stressing the heart, such as by infusing drugs such as adenosine or dobutamine, to determine how well the heart is being perfused by the coronary arteries. Stressing the heart can be advantageous in detecting ischemia because the coronary blood flow has to increase to the myocardium in order for the heart to work harder during the test. The increase in blood flow helps identify smaller defects in perfusion, i.e., helps increase the sensitivity or accuracy of the non-invasive test.
All of the current non-invasive stress tests involve significant time and expense. The most common non-invasive test ordered to rule on coronary ischemia, for example, is the SPECT test, which costs about $3,000 per test. Another common stress test, treadmill testing, requires a patient to run or walk on a treadmill while having sensors connected on various locations the patient's chest, involving significant time and energy for the patient. Further, current non-invasive tests are limited in their ability to detect certain types of arterial blockages, such as those caused by microvascular disease.
Over the last decade, the technique of placing a catheter lead into the coronary sinus for pacing (sometimes called cardiac resynchronization therapy or CRT) with an implantable cardioverter-defibrillator (ICD), i.e. a type of pacemaker that includes leads in both the right ventricle, the right atrial, and the coronary sinus, has become a standard of care in patients with symptoms of heart failure and a wide QRS, the ventricular component of an EKG. Indeed, the US ICD Registry Program estimates that approximately 10,000 ICD implants are placed into patients each month in the United States. Further, approximately 40% of the patients receiving ICDs are candidates for CRT (i.e. for having a lead placed in the coronary sinus). The leading cause of dilated hearts with widened QRSs is coronary ischemia.
Accordingly, a simple and cost effective mechanism for effectively detecting coronary ischemia, particularly for those patients with ICDs who are candidates for CRT therapy, is desired.